The present invention relates to a monitoring device in the wall of a furnace, particularly for capturing the image of an incineration process in the furnace, with the monitoring device having an inner end, an outer end, at least one housing, and a monitoring element arranged inside the housing, with the monitoring element extending between the inner end and the outer end. In a well-known monitoring device of the above-described type, compressed air is used to clean the inner end of deposits. If this does not succeed, then either manual cleaning must take place, which requires turning off the furnace, or the regulation of the incineration process must get along with less information or if faults remain undetected wrong information. In both cases this leads to additional costs.